Parsing an expression in Prolog and returning an abstract syntax

Question

I have to write parse(Tkns, T) that takes in a mathematical expression in the form of a list of tokens and finds T, and return a statement representing the abstract syntax, respecting order of operations and associativity.

For example,

?- parse( [ num(3), plus, num(2), star, num(1) ], T ).

T = add(integer(3), multiply(integer(2), integer(1))) ;
No

I've attempted to implement + and * as follows

parse([num(X)], integer(X)).
parse(Tkns, T) :-
  (  append(E1, [plus|E2], Tkns),
     parse(E1, T1),
     parse(E2, T2),
     T = add(T1,T2)
  ;  append(E1, [star|E2], Tkns),
     parse(E1, T1),
     parse(E2, T2),
     T = multiply(T1,T2)
  ).

Which finds the correct answer, but also returns answers that do not follow associativity or order of operations.

ex)

parse( [ num(3), plus, num(2), star, num(1) ], T ). 

also returns

mult(add(integer(3), integer(2)), integer(1))

and

parse([num(1), plus, num(2), plus, num(3)], T)

returns the equivalent of 1+2+3 and 1+(2+3) when it should only return the former.

Is there a way I can get this to work?

Edit: more info: I only need to implement +,-,*,/,negate (-1, -2, etc.) and all numbers are integers. A hint was given that the code will be structured similarly to the grammer

<expression> ::= <expression> + <term>
              |  <expression> - <term>
              |  <term>

      <term> ::= <term> * <factor>
              |  <term> / <factor>
              |  <factor>

    <factor> ::= num
              |  ( <expression> )

Only with negate implemented as well.

Edit2: I found a grammar parser written in Prolog (http://www.cs.sunysb.edu/~warren/xsbbook/node10.html). Is there a way I could modify it to print a left hand derivation of a grammar ("print" in the sense that the Prolog interpreter will output "T=[the correct answer]")

Answer 1

Before fixing your program, look at how you identified the problem! You assumed that a particular sentence will have exactly one syntax tree, but you got two of them. So essentially, Prolog helped you to find the bug!

This is a very useful debugging strategy in Prolog: Look at all the answers.

Next is the specific way how you encoded the grammar. In fact, you did something quite smart: You essentially encoded a left-recursive grammar - nevertheless your program terminates for a list of fixed length! That's because you indicate within each recursion that there has to be at least one element in the middle serving as operator. So for each recursion there has to be at least one element. That is fine. However, this strategy is inherently very inefficient. For, for each application of the rule, it will have to consider all possible partitions.

Another disadvantage is that you can no longer generate a sentence out of a syntax tree. That is, if you use your definition with:

?- parse(S, add(add(integer(1),integer(2)),integer(3))).

There are two reasons: The first is that the goals T = add(...,...) are too late. Simply put them at the beginning in front of the append/3 goals. But much more interesting is that now append/3 does not terminate. Here is the relevant failure-slice (see the link for more on this).

parse([num(X)], integer(X)) :- false.
parse(Tkns, T) :-
  (  T = add(T1,T2),
     append(E1, [plus|E2], Tkns), false,
     parse(E1, T1),
     parse(E2, T2),
  ;  false, T = multiply(T1,T2),
     append(E1, [star|E2], Tkns),
     parse(E1, T1),
     parse(E2, T2),     
  ).

@DanielLyons already gave you the "traditional" solution which requires all kinds of justification from formal languages. But I will stick to your grammar you encoded in your program which - translated into DCGs - reads:

expr(integer(X)) --> [num(X)].
expr(add(L,R)) --> expr(L), [plus], expr(R).
expr(multiply(L,R)) --> expr(L), [star], expr(R).

When using this grammar with ?- phrase(expr(T),[num(1),plus,num(2),plus,num(3)]). it will not terminate. Here is the relevant slice:

expr(integer(X)) --> {false}, [num(X)].
expr(add(L,R)) --> expr(L), {false}, [plus], expr(R).
expr(multiply(L,R)) --> {false}expr(L), [star], expr(R).

So it is this tiny part that has to be changed. Note that the rule "knows" that it wants one terminal symbol, alas, the terminal appears too late. If only it would occur in front of the recursion! But it does not.

There is a general way how to fix this: Add another pair of arguments to encode the length.

parse(T, L) :-
   phrase(expr(T, L,[]), L).

expr(integer(X), [_|S],S) --> [num(X)].
expr(add(L,R), [_|S0],S) --> expr(L, S0,S1), [plus], expr(R, S1,S).
expr(multiply(L,R), [_|S0],S) --> expr(L, S0,S1), [star], expr(R, S1,S).

This is a very general method that is of particular interest if you have ambiguous grammars, or if you do not know whether or not your grammar is ambiguous. Simply let Prolog do the thinking for you!

Answer 2

Removing left recursion will drive you towards DCG based grammars.

But there is an interesting alternative way: implement bottom up parsing.

How hard is this in Prolog ? Well, as Pereira and Shieber show in their wonderful book 'Prolog and Natural-Language Analysis', can be really easy: from chapter 6.5

Prolog supplies by default a top-down, left-to-right, backtrack parsing algorithm for DCGs.

It is well known that top-down parsing algorithms of this kind will loop on left-recursive rules (cf. the example of Program 2.3).

Although techniques are avail- able to remove left recursion from context-free grammars, these techniques are not readily generalizable to DCGs, and furthermore they can increase grammar size by large factors.

As an alternative, we may consider implementing a bottom-up parsing method directly in Prolog. Of the various possibilities, we will consider here the left-corner method in one of its adaptations to DCGs.

For programming convenience, the input grammar for the left-corner DCG interpreter is represented in a slight variation of the DCG notation. The right-hand sides of rules are given as lists rather than conjunctions of literals. Thus rules are unit clauses of the form, e.g.,

s ---> [np, vp].

or

optrel ---> [].

Terminals are introduced by dictionary unit clauses of the form word(w,PT).

Clearly, consider to complete the lecture, the book is freely available (see the last book entry in info page).

Now let's try writing a bottom up processor:

:- op(150, xfx, ---> ).

parse(Phrase) -->
    leaf(SubPhrase),
    lc(SubPhrase, Phrase).

leaf(Cat) --> [Word], {word(Word,Cat)}.
leaf(Phrase) --> {Phrase ---> []}.

lc(Phrase, Phrase) --> [].

lc(SubPhrase, SuperPhrase) -->
    {Phrase ---> [SubPhrase|Rest]},
    parse_rest(Rest),
    lc(Phrase, SuperPhrase).

parse_rest([]) --> [].
parse_rest([Phrase|Phrases]) -->
    parse(Phrase),
    parse_rest(Phrases).

% that's all! fairly easy, isn't it ?

% here start the grammar: replace with your one, don't worry about Left Recursion
e(sum(L,R)) ---> [e(L),sum,e(R)].
e(num(N)) ---> [num(N)].

word(N, num(N)) :- integer(N).
word(+, sum).

that for instance yields

phrase(parse(P), [1,+,3,+,1]).
P = e(sum(sum(num(1), num(3)), num(1))) 

note the left recursive grammar used is e ::= e + e | num

Answer 3

The correct approach is to use DCGs, but your example grammar is left-recursive, which won't work. Here's what would:

expression(T+E) --> term(T), [plus], expression(E).
expression(T-E) --> term(T), [minus], expression(E).
expression(T)   --> term(T).

term(F*T) --> factor(F), [star], term(T).
term(F/T) --> factor(F), [div], term(T).
term(F)   --> factor(F).

factor(N) --> num(N).
factor(E) --> ['('], expression(E), [')'].

num(N) --> [num(N)], { number(N) }.

The relationship between this and your sample grammar should be obvious, as should the transformation from left-recursive to right-recursive. I can't recall the details from my automata class about left-most derivations, but I think it only comes into play if the grammar is ambiguous, and I don't think this one is. Hopefully a genuine computer scientist will come along and clarify that point.

I see no point in producing an AST other than what Prolog would use. The code within parenthesis on the left-hand side of the production is the AST-building code (e.g. the T+E in the first expression//1 rule). Adjust the code accordingly if this is undesirable.

From here, presenting your parse/2 API is quite trivial:

parse(L, T) :- phrase(expression(T), L).

Because we're using Prolog's own structures, the result will look a lot less impressive than it is:

?- parse([num(4), star, num(8), div, '(', num(3), plus, num(1), ')'], T).
T = 4* (8/ (3+1)) ;
false.

You can show a more AST-y output if you like using write_canonical/2:

?- parse([num(4), star, num(8), div, '(', num(3), plus, num(1), ')'], T),
   write_canonical(T).
*(4,/(8,+(3,1)))
T = 4* (8/ (3+1)) a

The part *(4,/(8,+(3,1))) is the result of write_canonical/1. And you can evaluate that directly with is/2:

?- parse([num(4), star, num(8), div, '(', num(3), plus, num(1), ')'], T),
   Result is T.
T = 4* (8/ (3+1)),
Result = 8 ;
false.